Soil freezing-thawing processes on the tibetan plateau: A review based on hydrothermal dynamics

Abstract

The freezing and thawing processes of near-surface soil are one of the most significant physical characteristics of the land surface on the Tibetan Plateau (TP), as well as an essential index for estimating the existence and development of the permafrost and reflecting the climate change. The seasonal and diurnal freezingthawing processes of the near-surface soil cause the changes and even anomalies of water and energy balance between the land and the atmosphere on the TP, and thus significantly affect surface hydrological processes, ecological environment, carbon and nitrogen cycles, and the weather and climate system on the plateau and of the surrounding areas. This article discusses the observational and simulated changes and the impact on the climate by reviewing latest research progress in soil freezing-thawing processes (SFTPs) over the past 20 years. Our review shows that: 1) During a complete annual freezing-thawing cycle, each layer of soil generally experiences four stages: summer thawing period, autumn thawing- freezing period, winter freezing period, and spring thawingfreezing period. Due to the influence of local factors, the SFTPs show differences in the start and end dates, rate, and type of change. 2) Diurnal freezing-thawing cycles show large differences between the permafrost regions and the seasonally frozen regions, which are mainly reflected in the duration of diurnal freezing-thawing cycles. 3) Although different land surface models (LSMs) can well capture the spatiotemporal variations of physical quantity of SFTPs, all of them need to be revised according to the characteristic of LSMs of the TP. 4) Unreasonable freezing-thawing model parameterization schemes can be improved through avoiding the unstable iterative computation and determining the critical freezing-thawing temperature according to the thermodynamic equilibrium equation. According to the review of existing research, adding high-quality observation stations, using satellite remote sensing data to retrieve SFTPs and deepen the coupling of LSMs with regional climate models and global climate models, developing parameterization schemes that are suitable for SFTPs of the TP, and adjusting the model structures can be helpful for the simulation of SFTPs on the TP.